1. Field of the Invention
This invention relates to a method for selective recovery of cadmium from cadmium-bearing waste.
2. Description of the Prior Art
Such cadmium-bearing waste occurs in the electrical battery industry as waste from the manufacture of alkaline nickel-cadmium batteries and from such scrap batteries. The waste contains mainly the metals cadmium, nickel and iron, but small quantities of other metals may also occur. The predominant part of the waste is present in solid form. The invention is, however, not limited to use for waste from the battery industry but may also be used in respect to cadmium bearing waste from other fields.
Cadmium has earlier been extracted from battery waste by separating the cadmium-bearing waste from that containing nickel, when this has been possible. The cadmium content has thereafter been recovered from the cadmium-bearing waste by distillation. This method, however, is costly and pollutes the environment.
Another method, especially for mixed scrap of cadmium-nickel type, has been leaching with acid and precipitation by electrolyses. This method, too, is costly and requires careful supervision.
A process has also been proposed by means of which the waste is first washed and treated pyrometallurgically, and thereafter dissolved in an ammonium nitrate solution without dissolving nickel and iron. Cadmium is then precipitated as cadmium carbonate by the introduction of gaseous CO.sub.2 into the solution. The excess CO.sub.2 is expelled from the leaching solution and small quantities of leached nickel are separated by liquid extraction before the leaching solution is reused for renewed leaching. The disadvantage of this method, apart from the costly furnace apparatus, is that only about 94% of the cadmium is leached out of the waste and that the iron and the main quantity of the valuable nickel are not separated from one another. The CO.sub.2 gas expelled is also lost, which adds to the expense of the process.